1. Field of the Invention
The present invention relates to an image forming apparatus and a voltage generation circuit.
2. Description of the Related Art
Image forming apparatuses are known that employ an electrophotographic method or an electrostatic recording method. Such apparatuses use a one-component developer mainly composed of a magnetic toner or a two-component developer mainly composed of a non-magnetic toner and a magnetic carrier for a developing process. In particular, many image forming apparatuses that form a full color image or a multi-color image by the electrophotographic method use the two-component developer in view of the color quality and the like of images.
In the above-described developing process, a developing device develops an electrostatic latent image formed on an image carrier by use of a developer. At this time, a high voltage in which an alternating voltage is superimposed on a direct voltage is applied to the developing device.
If a high voltage is applied to a development gap (a gap between the image carrier (a photosensitive member) and the developing device (a developing sleeve)), a ring-like or a spot-like pattern as shown in FIGS. 6A and 6B may appear in a formed toner image (hereinafter referred to as a ring mark). FIG. 6A shows a ring mark that appears in a background portion, and FIG. 6B shows a ring mark that appears in an image portion. The occurrence of such a ring mark considerably impairs the image quality.
Here, a mechanism of occurrence of a ring mark will be described. As an image forming process is repeatedly performed in an image forming apparatus, the surface of the developing sleeve is scraped by the carrier or the like, or metal powder from a metal screw or the like intrudes into a developing device. If metal foreign matter intrudes into the developing device, the development gap narrows, and consequently, a discharge occurs between the metal foreign matter and the surface of the image carrier. The occurrence of a ring mark is due to this discharge phenomenon.
In light of the principle of occurrence of a ring mark as described above, a technique of inhibiting a ring mark that appears in the background portion by decreasing a positive voltage Vp+ of a developing alternating voltage shown in FIG. 7 is known. FIG. 7 shows an example of the surface potentials of the developing device and the image carrier and an outline of developer movement. Vdark indicates the surface potential of a region of the image carrier that has been uniformly charged by a charger and has not been exposed by an exposure unit (i.e., that is not to be developed). Vlight indicates the potential of a latent image formed on the image carrier by exposure. Vdc indicates a direct potential applied to the developing sleeve (the developing device) by a developing DC generator, and Vp+ and Vp− indicate amplitude values of an alternating voltage applied to the developing sleeve by a developing AC generator.
Here, the magnitude of a potential difference Vcontrast between Vdc and Vlight influences a development density, that is, the density of a visible image. Moreover, a potential difference Vback between Vdark and Vdc is a potential difference for preventing an unexposed portion from being developed (so-called fogging prevention). Decreasing Vp+ decreases the potential difference between Vdark and Vp+ and can result in a ring mark reduction.
Moreover, if a latent image of a high-density region and a latent image of a low-density region are adjacent to each other on the image carrier, a developer that should stick to the low-density region is attracted to the latent image of the high-density region. Consequently, a region that originally has to be developed fails to be developed, and a phenomenon in which an image is missing (hereinafter referred to as a white spot) may occur.
Here, it is known that at a large absolute value of Vp−, the potential difference between Vp− and Vlight is large, and the developability is improved even with respect to the latent image of the low-density region. Moreover, if an alternating voltage is applied to the developing sleeve, an electric field Vp− in a direction in which the developer is allowed to fly from the developing sleeve to the photosensitive member and an electric field Vp+ in a direction in which the developer is returned from the photosensitive member to the developing sleeve are alternately produced, so that the developer is oscillated so as not to cause a development unevenness. Therefore, the application of an alternating voltage is effective in rendering the development density uniform.
Moreover, a technique of using an asymmetric duty cycle blank pulse waveform when applying a voltage to the developing device as described above has been proposed. With this waveform, the time for which a positive voltage Vp+ is output and the time for which a negative voltage Vp− is output can be made different from each other. Moreover, the absolute value of Vp+ can be smaller than the absolute value of Vp−, and even a blank period can be set. Thus, demerits of the use of an alternating voltage can be suppressed while utilizing the above-described merits of a high alternating voltage (see Japanese Patent Laid-Open No. 2009-33815).
In applying a voltage to the developing device, especially in an operation sequence when starting output, a transformer provided in a high voltage generation circuit for generating the voltage to be applied may reach magnetic saturation. Magnetic saturation of a transformer means that the magnetic permeability of a core is 1, and occurs when the magnetic flux density in the transformer core exceeds a saturation magnetic flux density, which depends on the shape, material, and number of turns of the core. If the transformer reaches magnetic saturation, the inductance rapidly decreases, so that a large current flows through the transformer, leading to damage to the circuit. The magnetic flux density in the core is proportional to a current flowing through a coil of the transformer.
Here, the current is expressed as follows:I=∫V/Ldt V: voltage, L: inductance
That is to say, the magnetic flux density is proportional to the product (VT product) of the voltage input to the transformer and the time. Accordingly, for example, in order to prevent magnetic saturation due to an increased VT product, it is necessary to increase the number of turns of the transformer, increase the core size, change the core to a core material having a high saturation magnetic flux density, or take other measures. All of such measures involve an increase in the cost of the transformer.